There are no naturally occurring, silver-coloured, metallic gem materials with a precious metal, in particular platinum content sufficiently high to provide a high intrinsic value, and in particular, to be hallmarkable, and with a Mohs hardness greater than 5.
Pure platinum and platinum alloys with Vickers hardnesses between 50 and 300 are too soft to be cut, optionally faceted, and polished to form durable "gemstones". Synthetic and natural gemstones used in the production of watch casings, facings or dials generally have Moh's hardness greater than 5 and the commonly used sapphire (corundum) has a Mohs hardness of 9. This ensures good polish and suitable wear properties. Natural metallic gemstones such as haematite have Mohs hardnesses of 6.5 to 7.5.
Pure platinum and existing platinum alloys cannot approach these hardness values. Even existing hard platinum alloys are not sufficiently scratch resistant to prevent scratches tom dulling the polished surface.
Other methods of increasing the hardness of platinum involve surface treatments. Hardness values of up to 700 Vickers (Mohs hardness 6) have been reported, sufficiently hard to polish to a highly reflective surface finish. However, only a thin surface layer up to 200 micrometres thick is affected and polishing, finishing, wear etc could result in compromising the integrity of the layer. Furthermore, these surface layers are often brittle, prohibiting subsequent mechanical work.
Plating finished platinum articles can also give a highly reflective surface finish, but has the disadvantage that it can discolour, wear off and needs expertise if it is to be done well.
Coloured platinum intermetallic compounds have been described in the prior art in South African Patent No. 90/7777. This product, unfortunately, suffers from two disadvantages, namely:
(i) it does not produce a silver coloured material, and, PA0 (ii) the platinum content is too low to conform to most, if not all, hallmarking requirements.
In the case of platinum, relatively high platinum contents are required for hallmarking purposes such as, for example, in the case of Japan, not less than 85 weight percent; and in the case of the USA, not less than 95 weight percent platinum. In the USA an alternative is that there be 50% platinum by weight with the balance of the 95%, namely 45%, comprising other platinum group metals. In Europe, the requirement is commonly 95 weight percent platinum.